Generally one would like to use the abundances measured in an HII
region as a probe of the global ISM abundances. Often, one
has the opportunity to measure several HII regions in a galaxy,
and, thus, test this assumption.
The starting assumption is that the HII region is the result of the
photoionization of the surrounding ISM by one or more newly formed massive
stars. A second implicit assumption is that the ionized gas of the
HII region has not been enriched by stellar winds or ejecta from the
exciting star.

Exceptions have been observed in HII regions excited by single stars
in which ejected
stellar material is ionized by the exciting star. The signature of
this is enhanced N and He (typically factors of 5 - 10 and 20 percent
respectively) with an associated O under-abundance (typically a
factor of 2 - 5). These have been observed in our Galaxy (see
Esteban et al. 1992;
and references therein) and in the LMC
(Heydari-Malayeri,
Melnick, & van Drom 1990;
Garnett & Chu 1994).
Thus, while there is some observational evidence for local ``pollution''
associated with a individual WR star or a Luminous Blue Variable
star (e.g, Carina,
Davidson et al. 1986),
it does not appear to be
a large effect when observing nebulae excited by a large cluster of
stars.

We will return to this theme in Section 4.1,
where I review the Ph.D. thesis work of
Chip Kobulnicky (1997)
- an observational
exploration into this problem of pollution and mixing timescales
within the realm of irregular galaxies.